1. Field of the Invention
This invention relates to a process for producing high-purity nitrogen trifluoride (NF3) gas. In particular, it relates to a production process whereby high-purity nitrogen trifluoride gas can be industrially provided with a low cost.
2. Description of the Related Art
Nitrogen trifluoride gas has increasingly become important in applications related to an electronic material, particularly as a gas for dry-etching during manufacturing a semiconductor device, a gas for dry-cleaning of a plasma CVD apparatus or a gas for cleaning a batchwise production apparatus for a wafer type device in a liquid crystal display using TFT, and thus its production quantity has been considerably increased. It has been needed to provide higher-purity NF3 gas for the above applications
There have been proposed a variety of processes for producing NF3 via molten salt electrolysis. For example, a process using nickel as an anode is industrially common because it produces few impurities such as CF4.
Acid ammonium fluoride (ammonium hydrogenfluoride) is used as a material for the molten salt electrolysis. Ammonium hydrogenfluoride commercially available as a reagent contains ammonium hexafluorosilicate as an impurity in a significant amount. Thus, because of a lower amount of impurities, it is preferable to use ammonium hydrogenfluoride prepared from hydrofluoric acid and ammonia as described in JP-A 4-56789. However, since recent technical improvement increasingly requires higher-purity NF3 gas, it has been needed to provide the product gas with a higher purity.
Such a higher-purity gas may be obtained by purifying a crude gas produced after the molten salt electrolysis (hereinafter, referred to as a xe2x80x9ccrude gasxe2x80x9d); for example, by introducing it along with a carrier gas into a purifier or purifiers in which adsorption by, e.g., zeolite, activated alumina or silica gel, chemical cleaning, plasma decomposition, low-temperature separation and/or liquefied-gas rectification occur.
The crude gas contains, other than a carrier gas and moisture (H2O), a significant amount of various impurities such as dinitrogen monoxide (N2O), carbon dioxide (CO2), carbon monoxide (CO), dinitrogen difluoride (N2F2), oxygen difluoride (F2O), sulfur hexafluoride (SF6) and carbon tetrafluoride (CF4). Thus, the crude gas must be purified with the above purifier or purifiers.
When using the purifiers, it is necessary to control their performance depending on the levels of the impurities or their variation. For example, for an apparatus where adsorption occurs, various parameters must be adjusted by, e.g., altering an adsorption rate or a frequency of replacing or regenerating an adsorbent. Thus, it may require additional labor and complicated quality control such as frequent checking a purified product gas for its purity variation, leading to increase in a cost.
For industrially producing NF3 gas with a purity of 4N (99.99 vol %), 5N (99.999 vol %) or higher, controlling an impurity level in a crude gas, i.e., an impurity content, is quite important. There are instrumental and economical restrictions in purifying a crude gas containing a significant amount of impurities to provide a high purity gas. It is, therefore, substantially difficult to economically produce a high purity gas.
An object of this invention is to provide a process for conveniently producing high-purity nitrogen trifluoride gas with a purity of 4N or higher using a conventional purification method or apparatus by reducing an impurity content in a crude gas.
The present inventors have intensely investigated causes for generation of impurities to solve the above problems, and have found that impurities in nitrogen trifluoride gas are mainly derived from minor components contained in Ni used as an electrode and thus the impurity content can be controlled by using an electrode with a given purity to considerably reduce impurity gases and to produce high-purity NF3 gas.
This invention provides a process for producing high-purity nitrogen trifluoride gas by molten salt electrolysis using a nickel electrode and ammonium hydrogenfluoride as an electrolyte, wherein carbon constituting impurity gases entrained in a crude gas, among impurities in the nickel electrode as an anode is controlled to an amount of 400 wt ppm or less, preferably 200 wt ppm or less, more preferably 100 wt ppm or less.
The process of this invention is an extremely simple process where molten salt electrolysis is conducted using an electrode, particularly an anode, made of nickel with a given purity to industrially produce high-purity nitrogen trifluoride gas with an economical cost.
Controlling the impurity content in the nickel electrode allows high-purity nitrogen trifluoride gas which cannot be provided by the prior art, to be practically produced.